Hard disk drives (HDD) have become indispensable information storage apparatuses for computers and various consumer electronics products, particularly, for the purpose of large capacity information storage. Magnetic recoding systems are basically classified into two types of technical methods based on the direction of a magnetization vector in the magnetic recording layer in a magnetic recording medium: one is longitudinal magnetic recording, and the other is perpendicular magnetic recording. The perpendicular magnetic recording system is considered superior in high-density recording to the conventional longitudinal magnetic recording system, since its magnetization state of recorded information is stable due to a small demagnetizing field acting on a region near the magnetization transition region between recorded bits. In recent years, the recording systems of HDDs have been under transition from the longitudinal magnetic recording system to the perpendicular magnetic recording system. While the recording density attained by the longitudinal magnetic recording system is about 100 Gb/inch2, it has been demonstrated that a recording density higher than 300 Gb/inch2 can be attained by the perpendicular magnetic recording system.
IEEE Transactions on Magnetic, vol. 36, pg. 2393, (2000) (“Non-Patent Document 1”) and IEEE Transactions on Magnetics, vol. 38, pg. 1976, (2002) (“Non-Patent Document 2”) disclose the typical structures of the current perpendicular magnetic recording media. The perpendicular magnetic recording medium has two magnetic layers: a soft-magnetic underlayer and a magnetic recording layer. These magnetic layers are separated by a non-magnetic intermediate layer. The magnetic recording layer has a structure in which fine magnetic grains comprising a CoCrPt-based alloy with the crystal c-axis oriented in the direction perpendicular to the film surface are separated by a crystal grain boundary comprising an oxide or the like (granular structure). For the intermediate layer, a polycrystalline material having the same hexagonal closed pack structure (hcp) as that of a CoCrPt alloy is used for improving the crystal orientation of the magnetic recording layer, and Ru is used particularly preferably.
In the magnetic recording layer having the granular structure, since exchange interactions acting between the magnetic grains separated by the crystal grain boundary is smaller than those acting on the inside of each of the magnetic grains, the magnetic grain is a basic unit for magnetization reversal in magnetic recording. However, it is difficult to completely eliminate the exchange interactions between the magnetic grains, and the magnetization reversal unit in the magnetic recording layer tends to be larger than the average grain size of the magnetic grains. It is one of the main purposes in the development of the media to decrease the exchange interaction between the magnetic grains as small as possible and to improve the independency of each of the magnetic grains in the magnetization direction, thereby enabling high-density recording of information. At the same time, it is necessary to further scale down the magnetic grains and make the grain size uniform.
The intermediate layer has the role of not only improving the crystal orientation of the magnetic recording layer but also controlling the fine granular structure of the magnetic recording layer. It has been known that the fine structure of the magnetic recording layer formed above the intermediate layer changes greatly depending on the material and the formation method for the intermediate layer. Accordingly, design of the intermediate layer is extremely important for improving the high-density recording performance of the perpendicular magnetic recording medium, and various structures, materials, and manufacturing methods have been proposed so far for the intermediate layer.
Japanese Patent Publication No. 2002-197630 (“Patent Document 1”) discloses a perpendicular magnetic recording medium having a magnetic recording layer of a granular structure comprising magnetic grains of CoCrPt alloy crystals and a crystal grain boundary of an oxide and an intermediate layer comprising Ru. The intermediate Ru layer is formed in two steps, in which an Ru layer of 15 nm thickness is formed under low Ar gas pressure in the first step, and an Ru layer of 10 nm thickness is formed under high Ar gas pressure in the second step. By forming a magnetic recording layer above the intermediate layer thus formed, a high signal-to-noise ratio (SNR) can be attained. Journal of Applied Physics, vol. 97, pg. 10N119, (2005) (“Non-Patent Document 3”) discloses the result of close investigations into the fine structure of a perpendicular magnetic recording medium formed by a similar method. Ru layers formed under different film forming conditions have different fine structures; the first Ru layer has a closely packed structure, and the second Ru layer has a physically separated granular structure. By properly combining the first Ru layer and the second Ru layer, formation of the crystal grain boundary in the initial region of the magnetic recording layer can be promoted while attaining good crystal orientation of the magnetic recording layer. In this case, it appears that the first Ru layer mainly has the role of improving the crystal orientation, while the second Ru layer mainly has the role of promoting the grain boundary formation.
Japanese Patent Publication No. 2003-123239 (“Patent Document 2”) and Japanese Patent Publication No. 2003-178412 (“Patent Document 3”) disclose perpendicular magnetic recording media each having a magnetic recording layer with a granular structure and an intermediate layer comprising an alloy material formed by adding an additive to Ru. The intermediate layer is formed by using an Ru-based alloy formed by adding one or more materials selected from the group consisting of C, Cu, W, Mo, Cr, Ir, Pt, Re, Rh, Ta, and V to Ru. It is preferred that a seed layer having a face-centered cubic (fcc) crystal structure with its (111) axis oriented in the direction perpendicular to the film surface be also present below the intermediate layer. Examples of the seed layer include a metal or an alloy containing either one of Cu, Au, Pd, Pt, and Ir, or an alloy containing at least Ni and Fe (Ni—Fe—Cr, etc.). According to Patent Documents 2 and 3, since this structure attains improvement in the crystal orientation and the initial growth layer of the magnetic recording layer and at the same time attains decrease in the crystal grain size of the magnetic recording layer, improvement in the performance of the recording medium is attained. In this case, it appears that the seed layer mainly has the role of improving the crystal orientation and the Ru alloy material mainly has the role of promoting the grain boundary formation.
Japanese Patent Publication NO. 2002-334424 (“Patent Document 4”) and Japanese Patent Publication No. 2005-39040 (“Patent Document 5”) disclose intermediate layers in which oxides are added to Ru or an Ru alloy. To the oxides, an Si oxide, Al oxide, Zr oxide, Ti oxide, Hf oxide, etc. can be applied. Further, according to Patent Document 4, it is also possible to replace the Ru alloy with an Re alloy. Since such intermediate layers each have a structure in which the crystal grain boundary comprising the oxide surrounds the fine crystal grains comprising the Ru alloy, those intermediate layers interact well with the granular structure of the magnetic recording layer. Accordingly, it is possible to promote the formation of the oxide crystal grain boundary in the initial growth layer of the magnetic recording layer to attain the improvement in medium performance. According to Patent Documents 4 and 5, a seed layer for controlling the crystal orientation of the intermediate layer and the magnetic recording layer is preferably applied also to the intermediate layer. According to Patent Document 4, the seed layer can be formed of an alloy such as NiAl, FeAI, CoFe, CoZr, NiTi, AlCo, AlRu, and CoTi. Patent Document 5 gives examples of alloys such as RuCo, RuCr, and RuCoCr as the material applied to a layer corresponding to the seed layer.
In selecting the material of the intermediate layer, matching with the crystal lattice constant of the magnetic recording layer is also important. Japanese Patent Publication No. 2003-203330 (“Patent Document 6”) discloses that the crystal orientation of the magnetic recording layer are improved and segregation of the oxide to the grain boundary is promoted by improving the lattice matching between the intermediate layer and the magnetic recording layer. In this case, the intermediate layer can be constituted of a non-magnetic metal comprising at least one element of Ru, Os, or Re as a main ingredient. Patent Document 6 discloses that improvement in medium SNRs was observed by limiting mismatching of the a-axis lattice constant to no more than 6% and mismatching of the c-axis lattice constant to no more than 4% between the crystal lattices of the magnetic recording layer and the intermediate layer both having the hexagonal close packed (hcp) structure.